The present invention relates to seat belt pretensioners and, more specifically, to a seat belt pretensioner that incorporates a motion multiplier thereby reducing the actuator stroke requirement. Furthermore, the present invention provides pretensioner drive mechanisms that substantially simplify the pretensioner design as compared to known designs.
Seat belt pretensioners remove slack from a seat belt in the event of a collision in order to minimize forward movement of the passenger. While it is known to use pyrotechnic gas generators to operate mechanisms which wind up or otherwise pull in slack in the seat belt during a collision, such known pyrotechnic gas generators are generally disposed internally of a seat belt retractor. Thus, the vehicle owner is faced with a significant cost penalty in that the entire pretensioner and retractor assembly must be replaced after activation because of the inability to prevent degradation of the retractor. High-temperature gases tend to abrade interior metal surfaces and produce ash and clinkers that bind up the retraction mechanism.
Another problem with known pretensioners is that they are designed to activate only in severe accidents, for example, accidents that exhibit xe2x80x9cGxe2x80x9d forces sufficient to activate the vehicle airbags. Safety system designers generally choose such a relatively high activation threshold due to the expense of replacing the entire seat belt retractor and pretensioner assembly after activation. As a result, seat belt pretensioners do not protect passengers in less severe accidents.
A related problem with known pretensioners is that when the pretensioner is activated only in severe accidents, activation is relatively late in the crash sequence. Thus, the pretensioner must rapidly take up slack in the seat belt thereby potentially aggravating the passenger.
U.S. Pat. No. 5,967,440, herein incorporated by reference, describes a pretensioner and a conventional retractor for a safety belt system. When compared to the seatbelt retractor, the pretensioner is relatively complex and large, thus increasing the weight and spatial requirements of the seatbelt assembly.
U.S. Pat. No. 5,899,399, herein incorporated by reference, describes a state of the art pretensioner. The pretensioner is relatively complex and as such complicates the manufacturing process.
U.S. Pat. No. 5,944,350, herein incorporated by reference, describes a state of the art pretensioner. Again, the pretensioner is relatively large, thus increasing the weight and spatial requirements of the seatbelt assembly.
German Patent No. DE 10010379 A1, herein incorporated by reference, describes a pretensioner having an arcuate path for a multi-piece actuator (a plurality of cylinders) that drives a steel cord wrapped around a webbing reel hub and thereby effects pretensioning of an associated seat belt. The manufacturing of the pretensioner is more complex given the multi-piece actuator and given the arcuate channel that the actuator must traverse. Uniformity in channel width and effective sealing becomes more difficult with the use of an arcuate channel. Secondly, a noise reduction means is employed to prevent rattling during normal vehicle operation, thereby increasing manufacturing costs. Furthermore, the joint use of the multi-piece actuator with the steel cord results in a friction loss and a reduction in the load applied to the steel cord by the forward-most cylinder of the actuator. As a result, more force is required to drive the multi-piece actuator and pretension the seatbelt. One solution is described by the use of an intermediate element xe2x80x9c43xe2x80x9d for facilitating the smooth movement of the rotating elements along the arcuate channel. Consequently, without the use of the intermediate element xe2x80x9c43xe2x80x9d, a more robust pretensioner housing would be necessitated to withstand the relatively greater combustion pressure (produced by a greater amount of propellant) likely necessary to pretension the seatbelt in accordance with customer requirements.
Other pretensioner designs employ relatively complicated clutch assemblies for clutching a seat belt retractor axle associated therewith. Many known pretensioner drive systems are also relatively complex. U.S. Pat. Nos. 6,042,041, 5,842,344, 5,794,876, and 5,699,976, herein incorporated by reference, illustrate the complexity of known pretensioners. Simplification of known designs, therefore, is desirable given a resultant simplification of manufacturing requirements.
The aforesaid concerns are resolved, in accordance with a preferred constructed embodiment of the present invention, by a seat belt pretensioner that utilizes a motion multiplier within an essentially sealed housing thereby preventing retractor exposure to combustion gases and clinkers. The present design also prevents occupant exposure to the gases and solids formed upon combustion of a pyrotechnic composition contained within the pretensioner.
In a preferred embodiment of the invention, the pretensioner contains a housing, an annular hub having spaced circumferential protrusions integral thereto, a retractor shaft axially disposed within the annular hub thereby providing mechanical cooperation between the pretensioner and a seat belt retractor, a perforated strap having a plurality of apertures wherein each aperture at least corresponds to a respective protrusion of the hub, a piston that communicates with the strap upon pretensioner activation, a gas generant or pyrotechnic that upon combustion propels the piston, and an initiator that ignites the gas generant upon a signal from an accelerometer, for example. Upon ignition of the gas producing pyrotechnic, the piston is driven forward in an essentially linear path thereby tightening the strap about the hub as the perforations mesh with the hub protrusions. The seat belt spooled about a second end of the retractor shaft within the seat belt retractor is thus pretensioned.